1. Field of the Invention
The present invention relates to inkjet printing methods and inkjet ink sets using white inkjet ink
2. Description of the Related Art
In inkjet printing, tiny drops of ink fluid are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a mechanism for ejecting the drops image-wise. Printing is accomplished by moving a printhead across the ink-receiver or vice versa or both.
When jetting the inkjet ink onto an ink-receiver, the ink typically includes a liquid vehicle and one or more solids, such as dyes or pigments and polymeric binders. It will be readily understood that the optimal composition of such ink is dependent on the printing method used and on the nature of the ink-receiver to be printed. The ink compositions can be roughly divided in:                water-based, the drying mechanism involving absorption, penetration and evaporation;        solvent-based, the drying primarily involving evaporation;        oil-based, the drying involving absorption and penetration;        hot melt or phase change, in which the ink is liquid at the ejection temperature but solid at room temperature and wherein drying is replaced by solidification; and        UV-curable, in which drying is replaced by polymerization.        
It should be clear that the first three types of ink compositions are more suitable for a receiving medium that is more or less absorptive, whereas hot melt inks and UV-curable inks are usually printed on non-absorbent ink-receivers.
The behavior and interaction of a UV-curable ink on a substantially non-absorbing ink-receiver was found to be quite complicated compared to water-based inks on absorbent ink-receivers. In particular, a good and controlled spreading of the ink on the ink-receiver proved to be problematic and adhesion problems were sometimes observed on using different types of non-absorbing ink-receivers. The same problems have been observed when solvent based inkjet inks including a binder were jetted on different types of non-absorbing ink-receivers.
One way to approach these problems is to develop and use different ink sets for different types of substrates, but this is a not a preferred solution since changing inks in the printer and printhead is very time consuming and not really a viable solution for an industrial printing environment. Therefore, the general approach is to modify the surface chemistry of the ink-receiver either with a suitable surface layer coating or by a pre-treatment such as plasma or corona treatment.
Corona discharge treatment and plasma treatment increase the cost, complexity and maintenance of the equipment used to process the substrates. Substrates may contain significant impurities or irregularities that may interfere with the treatment of the substrate, and hence not result to the uniform spreading and adhesion of ink.
The other possibility of using the same inkjet ink set on different ink-receivers by application of a surface layer prior to jetting also increases the complexity of the inkjet printer. Generally, the surface layer is coated and dried or cured before jetting the inkjet ink as, for example, in the inkjet printing process in EP 1671805 A (AGFA) and US 2003021961 (3M), but it can also remain a wet, un-cured surface layer as in WO 00/30856 (XAAR).
A single composition of a surface layer suitable for all the different substrates is however not available. WO 2006/111707 (SUN CHEMICAL) discloses a process of inkjet printing in which: i) a primer is applied to a substrate material; ii) ink is inkjet printed onto the primed substrate; iii) a characteristic relating to print quality is evaluated; iv) the composition of the primer is adjusted in dependence on the evaluated characteristic relating to print quality; and v) the adjusted primer composition is applied to the substrate material and ink is inkjet printed onto the primed substrate material to give a printed product. Surface layers increase the thickness of an ink-layer, which may result in a different look-and-feel and reduced flexibility of the ink-layer.
Inkjet printing methods wherein inkjet inks are mixed with colorless liquids just prior to jetting have also been investigated.
U.S. Pat. No. 6,550,892 (KODAK) discloses a drop-on-demand inkjet printing system for delivering droplets of selectable-color ink to a receiver by mixing a colorless liquid ink with liquid inks of a different color and delivering the ink-mixture to the ejection chamber of a printhead. Also U.S. Pat. No. 6,050,680 (CANON) relates to an inkjet recording apparatus that can record images with a plurality of inks with different densities for each color by mixing of a first ink containing colorant and a second ink containing no colorant.
Instead of mixing colored inks, U.S. Pat. No. 4,614,953 (LAITRAM) discloses a color inkjet printing mechanism utilizing a single stream flow of ink by injecting solid dyes into a carrier fluid to form colored ink. The mechanism is capable of a wider range of color tonalities, due to the premixing capabilities, than is possible using dithering techniques with three colored inks.
All these inkjet printing methods are concerned with enhancing the color gamut by jetting color ink diluted with different amounts of a colorless liquid, but are silent on how ink-mixtures should be made for printing on different ink-receivers, and more particularly are silent on making white ink-mixtures.
Pigments with a high refractive index, such as titanium dioxide, have to be used in the white ink in order to obtain a sufficient opacity of the printed layer. Sedimentation of these dense particles in a low viscosity fluid, such as an inkjet ink, is a real challenge for ink formulators. Problems of clogging of inkjet printhead nozzles and poor storage stability of the ink are direct consequences of sedimentation and aggregation of white pigments due to the difference in specific gravity between pigment particles and the liquid medium of the ink.
Various approaches have been used trying to overcome these problems. One approach is to improve the dispersability. EP 1388578 A (DAINIPPON INK) discloses an ultraviolet-curable ink composition for inkjet recording including titanium oxide, a polymeric dispersant having a basic functional group, a photopolymerizable compound and a photopolymerization initiator, the titanium oxide is surface-treated with silica and alumina and the weight of the silica, which coexists with the titanium oxide, is larger than that of the alumina.
Another approach is designing particles which exhibit less sedimentation. U.S. Pat. No. 4,880,465 (VIDEOJET) discloses a non-pigmented white inkjet ink including hollow microspheres containing a central void region filled with a liquid capable of diffusing through the walls of the microspheres and have an inside diameter from about 0.1 to about 0.5 micron and an outside diameter from about 0.4 to about 1 micron. Sedimentation is drastically reduced but the opacity of such a printed white layer remains limited.
A third approach is the adaptation of the hardware involving an agitating device for reducing sedimentation, such as e.g. a stirrer in the supply vessel of the white ink. Stirring may cause air bubbles in the white ink which may lead to jetting problems.
It is desirable to be able to print white layers of consistent quality on a wide variety of ink-receivers using a state-of-the-art inkjet printer not requiring any complex or costly adaptation of the printer, wherein the white inkjet ink does not suffer from sedimentation.
Printing on a wide variety of different ink-receivers, including non-absorbing substrates such as glass, metal or polymeric surfaces, may deliver inconsistent image quality and adhesion problems of the ink to some of the ink-receivers. A change of substrate then necessitates a cumbersome change of inkjet ink sets, a second inkjet printer or some pre-treatment installation of the substrate, which are all not desirable for reasons of productivity.